This invention relates to supported stereorigid metallocene catalyst systems useful in the polymerization of ethylenically unsaturated compounds and, more particularly, to processes for the preparation of supported metallocene catalysts incorporating large pore silica supports.
Numerous catalyst systems for use in the polymerization of ethylenically unsaturated monomers are based upon metallocenes. Metallocenes can be characterized generally as coordination compounds incorporating one or more cyclopentadienyl (Cp) groups (which may be substituted or unsubstituted) coordinated with a transition metal through xcfx80 bonding. When certain metallocene compounds are combined with an activator or cocatalyst such as methylaluminoxane (MAO) and optionally an alkylation/scavenging agent such as trialkylaluminum compound, highly active polymerization catalysts are formed. Various types of metallocenes are known in the art. As disclosed, for example, in U.S. Pat. No. 5,324,800 to Welborn et al, they include monocyclic (a single cyclopentadienyl group), bicyclic (two cyclopentadienyl groups, as shown in Formula 1), or tricyclic (three cyclopentadienyl groups) coordinated with a central transition metal. Homogeneous or non-supported metallocene catalysts are known for their high catalytic activity especially in olefin polymerizations. Under polymerization conditions where polymer is formed as solid particles, these homogeneous (soluble) catalysts form deposits of polymer on reactor walls and stirrers. These deposits should be removed frequently as they prevent an efficient heat-exchange, necessary for cooling the rector contents, and cause excessive wear of the moving parts. In addition, solid particles formed by such homogeneous catalysts possess undesirable particle morphologies with low bulk densities which make them difficult to circulate in the reactor, limiting throughput and they are difficult to convey outside of the reactor. In order to resolve these difficulties several supported metallocene compounds have been proposed. As disclosed in Welborn et al, typical supports include inorganic oxides such as silica, alumina or polymeric materials such as polyethylene.
Metallocene compounds, whether supported or unsupported, can further be characterized in terms of stereoregular catalysts which result in polymerization of alpha olefins, such as propylene, to produce crystalline stereoregular polymers, the most common of which are isotactic polypropylene and syndiotactic polypropylene. In general, stereospecific metallocene catalysts possess at least one chiral center and the ligand structure (usually cyclopentadienyl-based) are conformationally restricted. Due to the fluxional nature of Cp-type ligands, it is common for at least one of the Cp ligands to be suitably substituted to impart some measure of stereorigidity. Such stereospecific metallocenes can include un-bridged bicycle compounds of the general formula bicyclic coordination compounds of the general formula:
(Cp)2MeQnxe2x80x83xe2x80x83(1) 
which are characterized by the isospecific metallocenes as described below and dicyclopentadienyl compounds of the general formula:
CpCpxe2x80x2MeQnxe2x80x83xe2x80x83(2) 
characterized by the syndiospecific metallocenes described below. In the aforementioned formulas, Me denotes a transition metal element and Cp and Cpxe2x80x2 each denote a cyclopentadienyl group which can be either substituted or unsubstituted wit hCpxe2x80x2 being different from Cp, Q is an alkyl or other hydrocarbyl or a halogen group and n is a number within the range of 1-3. In such instances stereorigidity can be provided through substituent groups which result in steric hindrance between the two cyclopentadienyl moieties, as described, for example in U.S. Pat. No. 5,243,002 to Razavi. Alternatively, the cyclopentadienyl groups are in a conformationally restricted relationship provided by a bridged structure between the metallocene rings (not shown in Formulas (1) and (2) above). It is sometimes advantageous to utilize metallocene compounds in which the two cyclopentadienyl moieties (same or different) are covalently linked by a so-called bridging group such as a dimethylsilylene group. The bridging group restricts rotation of the two cyclopentadienyl moieties and in many instances improves catalyst performance. Metallocenes containing such a bridging group are often referred to as stereorigid. While bridged metallocenes normally incorporate two cyclopentadienyl groups (or substituted cyclopentadienyl groups), bridged metallocenes incorporating a single cyclopetadienyl group which is bridged to a heteroatom aromatic group (both being coordinated with a transition metal) are also known in the art. For example, U.S. Pat. No. 5,026,798 to Canich discloses dimethylsilyl-bridged cyclopentadienyl-anilino or other heteroatom ligand structures with coordination to the transition metal being provided through the nitrogen atom of the anilino group as well as the cyclopentadienyl-group. Other common bridging groups include CR1R2, CR1R2CR2R3, SiR1R2SiR1R2 where the Ri substituents can be independently selected from H or a C1-C20 hydrocarbyl radical. Alternate bridging groups can also contain nitrogen, phosphorus, boron or aluminum.
As noted previously, isospecific and syndiospecific metallocene catalysts are useful in the polymerization of sterospecific propagation of monomers. Sterospecific structural relationships of syndiotacticity and isotacticity may be involved in the formation of stereoregular polymers from various monomers. Sterospecific propagation may be applied in the polymerization of ethylenically unsaturated monomers such as C3 to C20 alpha olefins which can be linear, branched or cyclic, 1-dienes such as 1,3-butadiene, substituted vinyl compounds such as vinyl aromatics, e.g., styrene, vinyl chloride, vinyl ethers such as alkyl vinyl ethers, e.g., isobutyl vinyl ether, or even aryl vinyl ethers. Stereospecific polymer propagation is probably of most significance in the production or polypropylene or isotactic or syndiotactic structure.
The structure of isotatic polypropylene can be described as one having the methyl groups attached to the tertiary carbon atoms of successive monomeric units falling on the same side of a hypothetical plane through the main chain of the polymer, e.g., the methyl groups are all above or below the plane. Using the Fischer projection formula, the stereochemical sequence of isotatic polypropylene can be described as follows: 
In Formula 3 each vertical segment indicates a methyl group on the same side of the polymer backbone. In the case of isotatic polypropylene the majority of inserted propylene units possess the same relative configuration in relation to its neighboring propylene unit. Another way of describing the structure is through the use of NMR. Bovey""s NMR nomenclature for an isotatic sequence as shown above is . . . mmmm . . . with each xe2x80x9cmxe2x80x9d representing a xe2x80x9cmesoxe2x80x9d dyad in which there is a mirror plane of symmetry between two adjacent monomer units, or successive pairs of methyl groups on the same said of the plane of the polymer chain. As is known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and subsequently the crystallinity of the polymer.
In contrast to the isotatic structure, syndiotactic propylene polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the chain lie on alternate sides of the plane of the polymer. In the case of syndiotactic polypropylene, the majority of inserted propylene units have opposite relative configuration relative to its neighboring monomer unit. Syndiotactic polypropylene using the Fisher projection formula can be indicated by racemic dyads with the syndiotactic rrr shown as follows: 
Bovey""s NMR nomenclature for a syndiotactic sequence as shown above is . . . rrr . . . with each xe2x80x9crxe2x80x9d representing a xe2x80x9cracemicxe2x80x9d dyad in which successive pairs of methyl groups are on the opposite sides of the plane of the polymer chain. Similarly, any deviation or inversion in the structure of the chain lowers the degree of syndiotacticity and subsequently the crystallinity of the polymer. The vertical segments in the preceding example indicate methyl groups in the case of syndiotactic or isotactic polypropylene. Other terminal groups, e.g. ethyl, in the case of poly1-butene, chloride in the case of polyvinyl chloride, or phenyl groups in the case of polystyrene and so on can be equally described in this fashion as either isotatic or syndiotactic.
Polypropylene resins can also be obtained in which the propylene units are inserted in a more or less random configuration. Such materials are referred to as atactic and as such, these polymers lack any signs of crystallinity as determined by common methods of X-ray diffraction (XRD), heat of fusion by Differential Scanning Calorimetry, or density. Such atactic polymers also tend to be more soluble in hydrocarbon solvents than polymers which possess some crystallinity. Syndiotactic polymers with sufficiently high levels of syndiotacticity and isotactic polymers with sufficiently high levels of isotacticity are semi-crystalline. Similarly this can be established by any technique known to those skilled in the art such as XRD, DSC or density measurements. It is common for polymers to be obtained as a mixture of highly stereoregular polymer and atactic polymer. In such instances, it is often useful to perform solubility testing, such as the mass fraction soluble in xylene or boiling heptane for instance to establish the amount of atactic polymer present. In most instances, atactic polymers are more soluble than the stereoregular counterparts and therefore the mass fraction soluble in hydrocarbons provides an indirect indication of the amount of atactic polymer present. While various other stereoregular or quasi-stereoregular polymer structures, such as hemiisotactic or stereoisoblock structures, are known, the principal stereoregular polymer configurations of interest are predominantly isotactic and predominantly syndiotactic polymers.
Catalysts that produce isotactic polyolefins are disclosed in U.S. Pat. Nos. 4,794,096 and 4,975,403. These patents disclose chiral, stereorigid metallocene catalysts that polymerize olefins to form isotactic polymers and are especially useful in the polymerization of highly isotactic polypropylene. As disclosed, for example, in the aforementioned U.S. Pat. No. 4,794,096, stereorigidity in a metallocene ligand is imparted by means of a structural bridge extending between cyclopentadienyl groups. Specifically disclosed in this patent are stereoregular hafnium metallocenes which may be characterized by the following formula:
Rxe2x80x3(C5(Rxe2x80x2)4)2HfQpxe2x80x83xe2x80x83(5) 
In Formula (5), (C5(Rxe2x80x2)4) is a cyclopentadienyl or substituted cyclopentadienyl group, Rxe2x80x2 is independently hydrogen or a hydrocarbyl radical having 1-20 carbon atoms, and Rxe2x80x3 is a structural bridge extending between the cyclopentadienyl rings. Q is a halogen or a hydrocarbon radical, such as an alkyl, aryl, alkenyl, alkylaryl, or arylalkyl, having 1-20 carbon atoms and p is 2.
Catalysts that produce syndiotactic polypropylene or other syndiotactic polyolefins and methods for the preparation of such catalysts are disclosed in U.S. Pat. Nos. 4,892,851 to Ewen et al and U.S. Pat. No. 5,807,800 to Shamshoum et al. These catalysts are also bridged stereorigid metallocene catalysts, but, in this case, the catalysts have a structural bridge extending between chemically dissimilar cyclopentadienyl groups and may be characterized by the formula:
Rxe2x80x3(CpRn) (CpRxe2x80x2m)MEQkxe2x80x83xe2x80x83(6) 
In Formula (6), Cp represents a cyclopentadienyl or substituted cyclopentadienyl ring, and R and Rxe2x80x2 represent hydrocarbyl radicals having 1-20 carbon atoms. Rxe2x80x3 is a structural bridge between the rings imparting stereorigidity to the catalyst. Me represents a transition metal, and Q a hydrocarbyl radical or halogen. Rxe2x80x2m is selected so that (CpRxe2x80x2m) is a sterically different substituted cyclopentadienyl ring than (CpRn). In Formula (6) n varies from 0-4 (0 designating no hydrocarbyl groups, i.e., no further substitution other than the bridging substituent on the cyclopentadienyl ring), m varies from 1-4, and k is from 0-3. The sterically different cyclopentadienyl rings produce a predominantly syndiotactic polymer rather than an isotactic polymer.
Like their isospecific counterparts, the syndiospecific metallocenes are used in combination with co-catalysts. One particularly useful class of co-catalysts are based on organoaluminum compounds which may take the form of an alumoxane, such as methylalumoxane or a modified alkylaluminoxane compound. Alumoxane (also referred to as aluminoxane) is an oligomeric or polymeric aluminum oxy compound containing chains of alternating aluminum and oxygen atoms, whereby the aluminum carries a substituent preferably an alkyl group. The exact structure of aluminoxane is not known, but is generally believed to be represented by the following general formula xe2x80x94(Al(R)xe2x80x94Oxe2x80x94)xe2x80x94m for a cyclic alumoxane, and R2Alxe2x80x94Oxe2x80x94(Al(R)xe2x80x94O) mxe2x80x94 AlR2 for a linear compound, wherein R independently each occurrence is a C1-C10 hydrocarbyl, preferably alkyl, or halide and m is an integer ranging from 1 to 50, preferably at least about 4. Alumoxanes are typically the reaction products of water and an aluminum alkyl, which in addition to an alkyl group may contain halide or alkoxide groups. Reacting several different aluminum alkyl compounds, such as, for example, trimethylaluminum and tri-isobutyl aluminum, with water yields so-called modified or mixed alumoxanes. Preferred alumoxanes are methylalumoxane and methylalumoxane modified with minor amounts of other higher alkyl groups such as isobutyl. Alumoxanes generally contain minor to substantial amounts of starting aluminum alkyl compound(s). Other cocatalysts include trialkyl aluminum, such as triethylaluminum (TEAL) or triisobutylaluminum (TIBAL) or mixtures thereof. Specifically disclosed in the ""851 patent is methylalumoxane and triethylaluminum (TEAL).
Bridged metallocene ligands having a dissimilar cyclopentadienyl groups can result from the reaction of 6,6-dimethyl fulvene with a substituted cyclopentadiene such as fluorene or substituted fluorene derivative to produce a ligand characterized by an isopropylidene bridge structure. Preferably, this ligand structure is characterized as having a bilateral symmetry such as indicated by the isopropylidene(cyclopentadienyl fluorenyl) structure as shown in Formula 9 of the aforementioned U.S. Pat. No. 5,807,800. As described in the Shamshoum et al ""800 patent, the bilateral symmetry of the ligand structure is indicated by the balanced orientation about the broken line representing a plane of symmetry extending generally through the bridge structure and the transition metal atom.
As disclosed in the aforementioned U.S. Pat. No. 5,324,800 to Welborn supported catalysts can be prepared by converting a soluble metallocene to a heterogenous catalyst by depositing the metallocene on a suitable catalyst support. Other supported catalysts are disclosed in U.S. Pat. Nos. 4,701,432 and 4,808,561, both to Welborn, U.S. Pat. No. 5,308,811 to Suga et al, U.S. Pat. No. 5,444,134, to Matsumoto, U.S. Pat. No. 5,719,241 to Razavi and the aforementioned U.S. Pat. No. 5,807,800 Shamshoum et al.
As described in the Welborn ""432 patent, the support may be any support such as talc, an inorganic oxide, or a resinous support material such as a polyolefin. Specific inorganic oxides include silica and alumina, used alone or in combination with other inorganic oxides such as magnesia, titania, zirconia and the like. Non-metallocene transition metal compounds, such as titanium tetrachloride, are also incorporated into the supported catalyst component. The inorganic oxides used as support are characterized as having an average particle size ranging from 30-600 microns, preferably 30-100 microns, a surface area of 50-1,000 square meters per gram, preferably 100-400 square meters per gram, a pore volume of 0.5-3.5 cc/g, preferably about 0.5-2 cc/g. Generally, the particle size, surface area, pore volume, and number of surface hydroxyl groups are said to be not critical to the Welborn procedure. Silica is often the support material of choice. Specifically disclosed in Welborn is a catalyst in which bis(cyclopentadienyl)zirconium dichloride (unbridged metallocene) is supported on a high surface area silica dehydrated in dry nitrogen at 600xc2x0 C. and characterized as Davison 952. The Welborn ""561 patent discloses a heterogeneous catalyst which is formed by the reaction of a metallocene and an alumoxane in combination with the support material. The support in Welborn ""561 is described similarly as the support in the Welborn ""432 patent. Various other catalyst systems involving supported metallocene catalysts are disclosed in U.S. Pat. Nos. 5,308,811 to Suga et al and 5,444,134 to Matsumoto. In both patents the supports are characterized as various high surface area inorganic oxides or clay-like materials. In the patent to suga et al, the support materials are characterized as clay minerals, ion-exchanged layered compounds, diatomaceous earth, silicates, or zeolites. As explained in Suga, the high surface area support materials should have volumes of pores having radii of at least 20 Angstroms. Specifically disclosed and preferred in Suga are clay and clay minerals such as montmorillonite. The catalyst components in Suga are prepared by mixing the support material, the metallocene, and an organoaluminum compound such as triethylaluminum, trimethylaluminum, various alkylaluminum chlorides, alkoxides, or hydrides or an alumoxane such as methylalumoxane, ethylalumoxane, or the like. The three components may be mixed together in any order, or they may be simultaneously contacted. The patent to Matsumoto similarly discloses a supported catalyst in which the support may be provided by inorganic oxide carriers such as SiO2, Al2O3, MgO, ZrO2, TiO2, Fe2O3, B2O2, CaO, ZnO, BaO, ThO2 and mixtures thereof, such as silica alumina, zeolite, ferrite, and glass fibers. Other carriers include MgCl2, Mg(O-Et)2, and polymers such as polystyrene, polyethylene, polypropylene, substituted polystyrene and polyarylate, starches and carbon. The carrier has a surface area of 1-1000 m2/g, preferably 50-500 m2/g, a pore volume of 0.1-5 cm3 g, preferably 0.3-3 cm3/g, and a particle size of 20-100 microns.
Of the various inorganic oxides used as supports, silica, in one form or another, is widely disclosed as a support material for metallocene catalysts. The aforementioned U.S. Pat. No. 5,719,241 to Razavi, while disclosing a wide range of inorganic oxides and resinous support materials, identifies the preferred support as a silica having a surface area between about 200 and 600 m2/g and a pore volume between 0.5 and 3 ml/g. Specifically disclosed is a support identified as Grace ""952 having a surface area of 322 m2/g. In preparing the supported metallocenes as described in Razavi, the silica is dried under a vacuum for three hours to remove water and then suspended in toluene where it is reacted with methylalumoxane for three hours at reflux temperature. The silica is washed three times with toluene to remove the unreacted alumoxane after which a solution of two metallocenes is added and the mixture stirred for an hour. The supernatant liquid is then withdrawn, and the solid support containing the metallocene is washed with toluene and then dried under vacuum. Silica characterized as Davison D-948 or Davison D-952 also appears as a conventional metallocene support. For example, U.S. Pat. No. 5,466,649 to Jejelowo discloses the use of dehydrated Davison D-948 silica as a support for various unbridged metallocenes used in conjunction with supported co-catalysts. U.S. Pat. No. 5,498,581 to Welch et al discloses silica for use as a support for either bridged or unbridged metallocenes in which the silica is treated with carbon monoxide, water, and hydrocyl groups. Specifically disclosed is the silica, Davison D-948, having an average particle size of 50 microns. Other silica-based supports are disclosed in U.S. Pat. Nos. 5,281,679 to Jejelowo, 5,238,892 to Chang, and 5,399,636 to Alt. The Chang and Jejelowo patents disclose the use of a silica support identified as Davison D-948, which is characterized as a amorphous silica gel containing about 9.7 wt. % water. As described in the Chang and Jejelowo patents, alumoxane is formed directly on the surface of the silica gel by direct reaction of an alkyl aluminum with silica gel which is undehydrated so as to ensure the conversion of the quantity of the alkyl aluminum to an alumoxane that has a high degree of oligomerization. The water-impregnated gel is characterized as having a surface range of 10-700 m2/g, a pore volume of about 0.5-3 cc/g, and an absorbed water content of from about 10-50 wt. % in the case of the Jejelowo patent and about 6-20 wt. % in the case of the Chang patent. The average particle size for the silica is described in Chang to be from 0.3-100 microns and in Jejelowo from about 10-100 microns. After the alumoxane silica gel component has been formed, the metallocene may be added to the wet slurry.
Other supported catalyst systems are disclosed in European Patent Application No. 96111719.9 (EPO 819706Al) to Shamshoum et al. Here a silica support such as described above is pretreated with an alumoxane, such as methylalumoxane followed by addition of a syndiospecific metallocene on the MAO-treated silica. The supported metallocene is used in conjunction with an organo-aluminum co-catalyst such as a monoalkyl or dialkyl aluminum halides as described previously, or trialkylaluminums such as trimethylaluminum, triethylaluminum or tri-isobutyl aluminum (TIBAL). In the supported catalyst disclosed in EPO819706, the silica support is a high surface area, small pore size silica which is first dried, slurried in a non-polar solvent, and then contracted with methylalumoxane in a solvent. The metallocene was then dissolved in a non-polar solvent, particularly the same as used as the solvent for the alumoxane. The solid metallocene supported on the alumoxane-treated silica is then recovered from the solvent, dried, and then incorporated into carrier liquid such as mineral oil. The Shamshoum EPA application also discloses a pre-polymerization step which can be used to decrease the aging time of the catalyst in the trialkyl aluminum or other aluminum co-catalyst.
Yet, other supported catalyst systems incorporating bridged metallocene catalysts are disclosed in U.S. Pat. No. 5,968,864 to Shamshoum et al. Here, catalyst efficiency is improved by preparation procedure in which a support such as silica is treated with alumoxane in a non-polar solvent such as toluene and contacted with a solution of a metallocene at a reduced temperature, preferably in the range of 0xc2x0 C. to xe2x88x9220xc2x0 C. The resulting solid is then washed with hexane and dried overnight at room temperature.
In accordance with the present invention, there is provided a process for the preparation of a silica-supported metallocene catalyst in which the metallocene and co-catalysts components can be tailored with respect to the particulate silica support to provide a supported catalyst system which can be isolated and stored in a mineral oil slurry for prolonged periods of time and then used in the production of stereoregular polymers while alleviating or eliminating problems associated with reactor fouling and undesirable polymer fines. The resulting supported catalyst provides good activity which can be maintained when the process is used to produce an isospecific or a syndiospecific supported catalyst.
In carrying out the invention, there is provided a particulate catalyst support material comprising silica particles impregnated with an alumoxane co-catalyst with at least one-half of the co-catalyst disposed within the internal pore volume of the silica. The support material is contacted with a dispersion of a metallocene catalyst in an aromatic hydrocarbon solvent. The metallocene solvent dispersion and the alumoxane-containing support are mixed at a temperature of about 10xc2x0 C. or less for a period sufficient to enable the metallocene to become reactively supported on and impregnated within the alumoxane-impregnated silica particles. Following the mixing time, which typically can vary from a few minutes to a number of hours, the supported catalyst is recovered from the aromatic solvent and then washed optionally with an aromatic hydrocarbon and then sequentially with a paraffinic hydrocarbon solvent in order to remove substantial quantities of aromatic solvent from the supported catalyst. These washing procedures are carried out at a low temperature of about 10xc2x0 C. or less. Thereafter, the washed catalyst is dispersed in a viscous mineral oil having a viscosity which is substantially greater than the viscosity of the paraffinic hydrocarbon solvent. Typically, the mineral oil is a viscosity at 40xc2x0 C. of at least 65 centistokes (units) as measured by ASTM D445. This may be contrasted with the viscosity of the paraffinic hydrocarbon solvent which usually will be no more than 1 centipoise at the reduced temperature conditions. Steps should not be taken to dry the washed catalyst, and typically the washed catalyst at the time of the dispersion will contain a substantial residual amount of the paraffinic hydrocarbon solvent. Preferably, after the supported catalyst is recovered from the aromatic solvent and before washing with the paraffinic hydrocarbon solvent, a further washing step is carried out with an aromatic solvent to remove unsupported metallocene from the supported catalyst.
In a further aspect of the invention, there is provided a particulate catalyst support comprising spheroidal silica particles having an average particle size within the range of 10-100 microns and an average effective pore diameter within the range of 200-400 Angstroms. Typically, the silica will be dried at an elevated temperature for a period of time to moderately dehydrate the silica. Often a mild heat treatment such as 100xc2x0 C. to 160xc2x0 C. is sufficient although higher temperatures can be employed. The silica support is than contacted with an alumoxane co-catalyst in an aromatic carrier liquid. The mixture of support, carrier liquid, and alumoxane co-catalyst is heated at an elevated temperature for a period of time to fix the alumoxane on the silica support with at least one-half of the alumoxane disposed internally within the silica support. For example, the mixture may be heated under reflux conditions of about 100xc2x0 C. or more for a period ranging from one hour to several hours. The mixture is then cooled and the alumoxane-containing support is separated from the carrier liquid. The alumoxane-containing support material is then washed with an aromatic hydrocarbon solvent in order to remove excess unsupported or free alumoxane (or aluminum alkyl residuals) so that substantially all of the alumoxane is fixed to the support. The alumoxane-containing support material is then cooled to a reduced temperature of about 10xc2x0 C. or less, and a dispersion of metallocene in an aromatic solvent is added with mixing as described above at a temperature of about 10xc2x0 C. or less to allow the metallocene to become reactively supported on and impregnated within the alumoxane-impregnated silica particles. The supported catalyst is then recovered, washed with a low viscosity paraffinic hydrocarbon solvent at a reduced temperature of about 10xc2x0 C. or less as described above and then dispersed in a viscous mineral oil. Alternatively, the catalyst is washed with mineral oil and no paraffinic hydrocarbon solvent is used. Polyolefin catalysts prepared in this fashion have superior performance qualities such as higher activity.